From which version of FileMaker did you first open this Invoices Starter solution?
When I look at the version that comes with FileMaker 13, Those fields are already on your layout without need to put them inside a pop over. (Though you can simply move the fields into a pop over frame if you want...)
I am using FileMaker 13, just recently downloaded.
Agreed those fields are there but I would like to add additional fields that i would like to have auto populated from the Customers database into the Invoices database.
Does that make sense?
It looked to me like the fields you were describing were already on your layout.
You can add a popover and just add the fields from Customer that you want directly to the popover.